The rear wall of the Regular Cab is 'vertical' (straight up), while the XtraCab rear wall is slanted forward.

Some topper manufacturers slant the front wall of their canopies / shells to match the XtraCab lines. Such 'slanted' XtraCab shells may hit the rear cab wall if fitted onto a Regular Cab. In any case, the lines of the rear cab wall and front shell wall won't match.

If you put a Regular Cab top onto an XtraCab, you might be left with a very noticeable gap between the forward-slanting cab and the straight-up canopy.